1. Field of the Invention
This invention relates to a non-destructive sugar content measuring apparatus applied to vegetables and fruits such as muskmelons, watermelons pumpkins and oranges and with which their sugar content can be measured without destroying the vegetables and fruits. More particularly, it relates to an improvement of the non-destructive sugar content measuring apparatus, which can measure the sugar content at an error within plus-minus 0.5 brix whenever the same vegetables and fruits kept at a constant temperature are measured (i.e., have measurement reproducibility).
2. Description of the Related Art
As methods for measuring the sugar content of vegetables and fruits without destroying them, methods are known in which near-infrared light or infrared light is made incident on vegetables and fruits to determine the sugar content by measuring the absorption of light by sugar on the basis of reflected light or transmitted light of the incident light (see Japanese Patent Applications Laid-open No. 1-216265, No. 1-235850, No. 2-147940, No. 4-104041, No. 4-208842, No. 5-34281, No. 5-172549 and No. 6-15236).
These methods, however, all make use of halogen lamps as light sources. Hence, light intensity is insufficient for vegetables and fruits having a thick rind, and it has been difficult to measure their sugar content.
Accordingly, as a method which can eliminate such a disadvantage, the applicant has already proposed a method in which three laser beams having different wavelengths are applied and at the same time the respective laser beams are successively made incident on vegetables and fruits such as muskmelons, watermelons pumpkins and oranges at substantially the same positions on their rinds.
To describe this measuring method briefly, as shown in FIG. 17 a laser beam having a wavelength xcex is made incident on a fruit M such as a muskmelon, e.g., from its lower side, and also the laser beam having a wavelength xcex which is emergent from the fruit M is detected with a detector (not shown) disposed similarly at the lower side. In FIG. 17, Pin(xcex) indicates the amount of incident light of the laser beam, and Pout(xcex) the amount of detected light of the laser beam.
Then, the sugar content (brix) of the fruit M can be determined from data of input signals Pin(xcex)xe2x80x2 corresponding to the amount of incident light Pin(xcex) and output signals Pout(xcex)xe2x80x2 corresponding to the amount of detected light Pout(xcex) and according to the following Equation (4).
More specifically, sugar content Y (brix) can be, in respect of the laser beams having three kinds of wavelengths, determined by substituting absorbance X(xcex):
X(xcex)=xe2x88x92log T(xcex)xe2x80x83xe2x80x83(2) 
which is a natural logarithmic value of transmittance T(xcex) defined as:
T(xcex)=Pout(xcex)/Pin(xcex)=(e2/e1)Pout(xcex)xe2x80x2/Pin(xcex)xe2x80x2xe2x80x83xe2x80x83(3) 
for Equation (4):
Y=AX(xcex1)+BX(xcex2)+CX(xcex3)+Dxe2x80x83xe2x80x83(4) 
Here, A, B and C are constants which are determined on many fruits (samples) by, e.g., the method of least squares in such a way that the correlation comes to be highest between sugar content Y determined with a refraction saccharometer and absorbance X(xcex1), X(xcex2) and X(xcex3) determined by photometry.
Incidentally, the amount of incident light Pin(xcex) and the output signals Pin(xcex)xe2x80x2 corresponding thereto are correlated as Pin(xcex)=e1xc2x7Pin(xcex)xe2x80x2, and the amount of incident light Pout(xcex) and the output signals Pout(xcex)xe2x80x2 corresponding thereto are correlated as Pout(xcex)=e2xc2x7Pout(xcex)xe2x80x2. Also, coefficients e1 and e2 are determined when the measuring apparatus is produced.
Then, from the values of electric signals Pin(xcex)xe2x80x2 and Pout(xcex)xe2x80x2, the sugar content Y (brix) is determined in the manner as described above.
To make description according to a conventional non-destructive sugar content measuring apparatus embodying this method, as shown in FIG. 18 the fruit M such as a muskmelon is placed on a tray d having two tray-side light passages g and h at its bottom and is transported. A laser beam having the amount of incident light Pin(1)is made incident on the fruit M from the leading end of an optical fiber w at each measuring section k provided in a transport path and having two measurement-side light passages i and j positionally adjusted to the tray-side light passages g and h of the tray d, through the measurement-side light passage i and tray-side light passage g. At the same time, the laser beam which has become emergent from the fruit M is made to enter each detector c through the tray-side light passage h and measurement-side light passage j to measure the output signals Pout(1)xe2x80x2 corresponding to the amount of detected light Pout(1), where the sugar content is measured on the basis of these output signals Pout(1)xe2x80x2 and input signals Pin(1)xe2x80x2 corresponding to the amount of incident light Pin(1).
Incidentally, in FIG. 18, m denotes a linear projection which is so provided as to extend in the lengthwise direction in the transport path at the measuring section k and prevents the laser beam from entering the measurement-side light passage j when it passes through the measurement-side light passage i; n, a linear recession which is provided at the bottom (under side) of the tray d and is loosely fitted to the linear projection m; p, a first side bar as a delivery position control means for controlling delivery position of the tray d at each measuring section k; q, a second side bar as a delivery position control means for pressing the tray d to the first side bar p side to control delivery position of the tray d; and 300, a laser output monitoring means whose main part is constituted of an output monitoring detector 3a, a distributor 3b and a light diffusion plate 3c. 
Now, when the sugar content of vegetables and fruits is measured by means of the non-destructive sugar content measuring apparatus of this type, the non-destructive sugar content measuring apparatus is demanded to measure the sugar content at an error within plus-minus 0.5 brix whenever the same vegetables and fruits kept at a constant temperature are measured, i.e., to have measurement reproducibility.
As factors which may obstruct this measurement reproducibility, what have conventionally been regarded are, e.g., a delicate change in laser output given from each laser light source, a poor ratio of noise to signal intensity (s/n ratio) at the time the light having transmitted through the interior of a fruit is converted into signals, and a roll of the tray or a sway of the fruit during measurement.
However, at any effort to devise the delivery position control means, the laser output monitoring means and so forth so as to remove such factors which may obstruct the measurement reproducibility, there has remained the problem that the measurement reproducibility in non-destructive sugar content measuring apparatus is still unsatisfactory.
Accordingly, the present inventors continued extensive studies in order to solve this problem. As a result, it has become revealed that, as a factor which may obstruct the measurement reproducibility of non-destructive sugar content measuring apparatus, a slight fluctuation in wavelength of laser beams emitted from laser light sources unexpectedly affects the accuracy of measurement to cause an error, in addition to the above various factors.
The present invention was made taking account of such a problem. Accordingly, an object of the present invention is to provide a non-destructive sugar content measuring apparatus which can measure the sugar content at an error within plus-minus 0.5 brix whenever the same vegetables and fruits kept at a constant temperature are measured (i.e., have measurement reproducibility).
Another object of the present invention is to provide a non-destructive sugar content measuring apparatus in which wavelengths xcex1, xcex2 and xcex3 of laser beams are selected from within the ranges of from 860 nmxe2x89xa6wavelength xcex1 less than 900 nm, 900 nmxe2x89xa6wavelength xcex2xe2x89xa6920 nm, and 920 nm less than wavelength xcex3xe2x89xa6960 nm, and which can measure the sugar content at an error within plus-minus 0.5 brix whenever the same vegetables and fruits kept at a constant temperature are measured.
Still another object of the present invention is to provide a non-destructive sugar content measuring apparatus in which wavelengths xcex1, xcex2 and xcex3 of laser beams are selected from within the ranges of from 800 nmxe2x89xa6wavelengths xcex1 and xcex2 less than 900 nm, and 900 nmxe2x89xa6wavelength xcex3xe2x89xa6920 nm (provided that the wavelengths xcex1 and xcex2 have a wavelength distance between them of 10 nm or larger), and which can measure the sugar content at an error within plus-minus 0.5 brix whenever the same vegetables and fruits kept at a constant temperature are measured.
To achieve the above objects, the present invention is, as a first embodiment, a non-destructive sugar content measuring apparatus comprising a plurality of trays on which vegetables and fruits are to be placed, a transport means for successively delivering the trays at appropriate intervals, and first, second and third measuring sections provided in the course of a transport path and at which laser beams having wavelengths xcex1, xcex2 and xcex3 are respectively made incident on each vegetable or fruit and the amount of light of each laser beam emergent from the vegetable or fruit is measured with a detector provided at each measuring section and at the same time the absorbance of each laser beam is determined from the amount of incident light made incident on the vegetable or fruit and the amount of detected light which has been measured with the detector, to measure the sugar content of the vegetables and fruits on the basis of each absorbance thus determined wherein;
the wavelengths xcex1, xcex2 and xcex3 of the laser beams satisfy the conditions of:
860 nmxe2x89xa6wavelength xcex1 less than 900 nm, 
900 nmxe2x89xa6wavelength xcex2xe2x89xa6920 nm, 
xe2x80x83920 nm less than wavelength xcex3xe2x89xa6960 nm,
and, where standard deviations of wavelength variations in the wavelengths xcex1, xcex2 and xcex3 are represented by xcex94xcex1, xcex94xcex2 and xcex94xcex3, respectively, satisfy the condition of the following mathematical expression (1):
[f1(xcex1)xc3x97xcex94xcex1+f2(xcex2)xc3x97xcex94xcex2+f3(xcex3)xc3x97xcex94xcex3] less than 0.5 brixxe2x80x83xe2x80x83(1). 
in which mathematical expression (1), f1(xcex1), f2(xcex2) and f3(xcex3) respectively represent sugar content variation functions (brix/nm) determined from sugar content variation curves showing the relationship between the wavelength variations in the wavelengths xcex1, xcex2 and xcex3 and the sugar content variations that accompany the former.
The present invention is, as a second embodiment, a non-destructive sugar content measuring apparatus comprising a plurality of trays on which vegetables and fruits are to be placed, a transport means for successively delivering the trays at appropriate intervals, and first, second and third measuring sections provided in the course of a transport path and at which laser beams having wavelengths xcex1, xcex2 and xcex3 are respectively made incident on each vegetable or fruit and the amount of light of each laser beam emergent from the vegetable or fruit is measured with a detector provided at each measuring section and at the same time the absorbance of each laser beam is determined from the amount of incident light made incident on the vegetable or fruit and the amount of detected light which has been measured with the detector, to measure the sugar content of the vegetables and fruits on the basis of each absorbance thus determined wherein;
the wavelengths xcex1, xcex2 and xcex3 of the laser beams satisfy the conditions of:
800 nmxe2x89xa6wavelengths xcex1, xcex2 less than 900 nm, 
900 nmxe2x89xa6wavelength xcex3xe2x89xa6920 nm, 
the wavelengths xcex1 and xcex2 having a wavelength distance between them of 10 nm or larger; and, where standard deviations of wavelength variations in wavelengths xcex1, xcex2 and xcex3 are represented by xcex94xcex1, xcex94xcex2 and xcex94xcex3, respectively, satisfy the condition of the following mathematical expression (1):
[f1(xcex1)xc3x97xcex94xcex1+f2(xcex2)xc3x97xcex94xcex2+f3(xcex3)xc3x97xcex94xcex3] less than 0.5 brixxe2x80x83xe2x80x83(1). 
in which mathematical expression (1), f1(xcex1), f2(xcex2) and f3(xcex3) respectively represent sugar content variation functions (brix/nm) determined from sugar content variation curves showing the relationship between the wavelength variations in the wavelengths xcex1, xcex2 and xcex3 and the sugar content variations that accompany the former.